Humans are the only host of norovirus, and norovirus has several mechanisms that allow it to spread quickly and easily. Norovirus infects humans in a pathway like the influenza virus’ mode of infection. In addition to their similar infective pathways, norovirus and influenza also evolve to avoid the immune system in a similar way. Both viruses are driven by heavy immune selection pressure and antigenic drift, allowing evasion of the immune system, which results in outbreaks. Norovirus can survive a wide range of temperatures and in many different environments. Moreover, the viruses can spread quickly, especially in places where people are in proximity, such as cruise ships and airline flights, even those of short duration.
Norovirus causes nearly 60% of all foodborne illness outbreaks. Norovirus is transmitted primarily through the fecal-oral route, with fewer than 100 norovirus particles needed to cause infection. Transmission occurs either person-to-person or through contamination of food or water. CDC statistics show that food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.
Infected individuals shed the virus in large numbers in their vomit and stool, shedding the highest number of viral particles while they are ill. Aerosolized vomit has also been implicated as a mode of norovirus transmission. Previously, it was thought that viral shedding ceased approximately 100 hours after infection; however, some individuals continue to shed norovirus long after they have recovered from it, in some cases up to 28 days after experiencing symptoms. Viral shedding can also precede symptoms, which occurs in approximately 30% of cases. Often, an infected food handler may not even show symptoms. In these cases, people can carry the same viral load as those who do experience symptoms.
Common settings for norovirus outbreaks include restaurants and events with catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%). Proper hand washing is the best way to prevent the spread of norovirus.
The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys this virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.
In 2006, there was a large increase in the number of norovirus cases on cruise ships. Norovirus cases were increasing throughout Europe and the Pacific at the same time. One issue with cruise ships is the close contact between people as living quarters are so close, and despite education efforts, there still seems to be a lack of public understanding regarding how the illness is spread. On the other hand, reporting occurs much more quickly in these situations because of the proximity and concentration of illness, allowing for the quicker detection of outbreaks. Cruise ship outbreaks often occur when new strains of norovirus are appearing, providing a good indicator system for new norovirus strains.
Finally, and as briefly mentioned earlier, outbreaks of norovirus infections have become synonymous with cruise ships. Healthcare facilities also experience a high incidence of norovirus outbreaks. The CDC has published information regarding the prevention of norovirus outbreaks on cruise ships and in healthcare facilities on its website. Once a case has occurred, even more stringent hygienic measures than normal are required to prevent an outbreak, particularly on an enclosed space such as a cruise ship.